1. Field of the Invention
The invention relates to an attenuating element which utilizes a conductor located between bonded yttrium-iron-garnet (YIG) ferrites. More particularly the invention relates to an improved attenuating unit and method of fabrication with the conductor closely coupled to the ferrites.
2. Description of the Prior Art
Frequency selective limiters (FSL) are attenuating devices which utilize a yttrium-iron-garnet (YIG) material. FSLs attenuate higher power level signals while simultaneously allowing lower power level signals, separated by only a small frequency offset from the higher level signals, to pass with relatively low loss. YIG based FSLs are capable of limiting or attenuating across more than an octave bandwidth in the 2-8 GHz range.
A portion of a fully assembled FSL is illustrated in prospective in FIG. 1. Signal carrying conductor 12 is positioned between first and second planar YIG layers 14 and 16. The YIG layers 14 and 16 are bonded by nonconductive adhesive 18. The second YIG layer 16 usually has an overall length less than the overall length of the first layer 14. As a result, the end portion 19 (one shown) of the signal carrying conductor 12 extends outwardly beyond the transverse edge 20 of the YIG layer 16 and thereby forms a wire bonding site The lower YIG layer 14 has a metallized surface 22. The YIG layers 14 and 16 and the signal carrying conductor 12 are supported on a metallized surface 24 of substrate 26 by an intermediate layer of conductive epoxy 28. The device illustrated in FIG. 1 is described in greater detail in a U.S. Pat. No. 4,845,439 issued Jul. 4, 1989 entitled "Frequency Selective Limiting Device", in the name of Steven N. Stitzer et al. and assigned to Westinghouse Electric Corporation, the assignee herein.
The YIG layers 14 and 16 are bonded together in confronting relationship by the nonconductive adhesive 18. The conductor carried by YIG layer 14 is sandwiched between the YIG layers 14 and 16. The glue thickness t.sub.g results in a gap 30 shown in fragmentary detailed cross section of FIG. 1A. Even with good control, the glue thickness t.sub.g may be equal to the conductor 12 thickness, e.g. 0.10 mils. Thus, the gap 30 separating the conductor 12 from the upper YIG layer 16 may be sufficiently large such that coupling efficiency is significantly diminished.
A processing technique used for making an individual FSL unit 10 is described in the U.S. Pat. No. 4,970,775. In the technique described in Brown et al., a planar ferrite member is secured to a metallized surface of the substrate. Thereafter, a plurality of linear signal carrying conductors are placed on the ferrite member in spaced relation. A second ferrite member is then bonded to the conductors and the first ferrite member with a nonconductive adhesive to form a multilayer structure. Grooves are cut into the multilayer structure between the adjacent conductors and extend through both ferrite members exposing the metallized surface of the substrate. The upper surface and the grooves of the multilayer structure are metallized in a conformal manner to form a ground plane and the individual units are then separated into a plurality of individual FSL units.
It has been found that the nonconductive adhesive used bond ferrite members is difficult to apply. In particular, the thickness of the nonconductive adhesive 18 is difficult to control and may vary greatly.
The spacing between the ferrites has a strong influence on performance. From an electrical standpoint, the ideal glue thickness should be zero. However, it has been found that when the spacing between the YIG layers is increased from about 0.10 mils to about 1.0 mils, the limiting effect of the device decreases from about 14 db to about 6db which is unsatisfactory.
In pending U.S. patent application entitled "Coplanar Waveguide Frequency Selective Limiter", Ser. No. 07/414,877, filed Sep. 29, 1989 now U.S. Pat. No. 4,980,657 in the name of Stitzer et al., and assigned to Westinghouse Electric Corporation, an FSL with coplanar ground planes is described. The device has ferrites bonded with nonconductive adhesive.
In the arrangements described, secure deposition of the conformal ground plane is difficult to achieve reliably because the nonconductive adhesive sometimes interferes with the deposition of the metal in the seam 32 between the ferrites 14 and 16. If the ground plane 24 is not fully adhered, RF leakage and interference may occur. This is also an unsatisfactory result.